Cu,Zn superoxide dismutase structure from a microbial pathogen establishesa class with a conserved dimer interface

Macrophages and neutrophils protect animals from microbial infection in par t by issuing a burst of toxic superoxide radicals when challenged. To count eract this onslaught, many Gram-negative bacterial pathogens possess peripl asmic Cu,Zn superoxide dismutases (SODs), which act on superoxide to yield molecular oxygen and hydrogen peroxide. We have solved the X-ray crystal st ructure of the Cu,Zn SOD from Actinobacillus pleuropneumoniae, a major porc ine pathogen, by molecular replacement at 1.9 Angstrom resolution. The stru cture reveals that the dimeric bacterial enzymes form a structurally homolo gous class defined by a water-mediated dimer interface, and share with all Cu,Zn SODs the Greek-key beta-barrel subunit fold with copper and zinc ions located at the base of a deep loop-enclosed active-site channel. Our struc ture-based sequence alignment of the bacterial enzymes explains the monomer ic nature of at least two of these, and suggests that there may be at least one additional structural class for the bacterial SODs. Two metal-mediated crystal contacts yielded our C222(1) crystals, and the geometry of these s ites could be engineered into proteins recalcitrant to crystallization in t heir native form. This work highlights structural differences between eukar yotic and prokaryotic Cu,Zn SODs, as well as similarities and differences a mong prokaryotic SODs, and lays the groundwork for development of antimicro bial drugs that specifically target periplasmic Cu,Zn SODs of bacterial pat hogens. (C) 2000 Academic Press.